Hot melt adhesives are applied to a substrate while in its molten state and cooled to harden the adhesive layer. Such adhesives are widely used for various commercial and industrial applications such as product assembly and packaging, and have been widely used in the non-woven industry to make baby diapers and adult incontinence products. In these applications, adhesive is applied to at least one substrate such as, for example, a film substrate (e.g., polyethylene), a non-woven substrate (e.g., polyolefin), or an elastic substrate (e.g., spandex) for binding the substrate to a second similar or different substrate.
Hot melt adhesives based on styrenic block copolymers have been used commercially as base polymers for hot melt adhesives and have found wide-spread use in nonwoven construction applications, e.g., disposable absorbent articles such as diapers, feminine hygiene articles, and adult incontinence devices. These products are typically applied at temperatures above 130° C. and often above 150° C. Lowering the application temperature of adhesives used in the manufacture of such products below 120° C. would improve the thermal aging in application equipment and reduce issues with heat sensitive or lower basis weight substrates. However, in order to apply the adhesive below 120° C. using current application technology, the viscosity must be low enough to spray and extrude cleanly. To lower viscosity, lower molecular weight polymers and higher levels of diluent have been used at the expense of performance of the hot melt adhesive. These approaches result in lower mechanical strength and more importantly less resistance to flow at elevated temperature. While the use of waxes to act as diluents as well as crystalline reinforcing agents is known, this approach suffers from a reduction in the effective open time the adhesive is capable of forming adequate bonds in the laminating processes used to make disposable articles.
There is a continuing need for a hot melt adhesive that can be applied at low temperature, i.e., below about 120° C., which has adequate open time for bonding and has a high resistance to flow at elevated temperatures. Such attributes would make the adhesives particularly well-suited for use in the manufacture of disposable articles. The invention is directed to this need.